Fuel is the largest operating cost for a typical truck fleet, and much effort has gone into improving the fuel economy of heavy trucks. One common configuration for a long-haul tractor trailer is a 6×4 tractor, consisting of one steer axle and two drive axles, pulling a trailer comprising two trailer axles. As a fuel-saving measure the 6×2 tractor has been introduced to the market. The 6×2 tractor consists of a steer axle, a non-driven “tag” axle, and a single drive axle. This eliminates one of the differentials from the drivetrain, simplifies the tag axle, reduces mass and drivetrain friction, and significantly improves fuel economy. However, drive traction is reduced since the number of tires transmitting the engine torque to the road has been reduced by half. This is usually only an issue at very low speeds in low gear on low traction surfaces.
To address this need for greater traction, axle manufacturers have introduced a system based on a 6×2 lift axle. A lift axle is capable of transferring load from the tag axle to the drive axle, increasing traction on the drive tires. The amount of load on the drive axle can be determined, e.g., by sensors. Means known in the art for measuring this load are described by U.S. Pat. No. 5,193,063, col. 6, lines 25 thru 45 and include placing a load cell between the axle and its suspension, placing a strain gauge between the axle and its suspension point, and measuring the pressure in the air springs when they are employed using a pressure transducer. Using suitable means for measuring the load, the tag axle can be lifted until the drive axle load reaches a predetermined value. The system then returns to normal loading based on various criteria. For example, the load may return to normal either when the vehicle speed exceeds a predetermined (low) threshold, a predetermined time passes, or the driver manually turns off the system. The drive axle may be loaded beyond the usual max load of the tires, as permitted by the T&RA tables of “Load and Pressure Adjustments at Reduced Speeds”. For example, the drive axle may be loaded to 20,000 lbs or 26,000 lbs at very low speeds for startup traction.
The means used for lifting or lowering the lift axle include hydraulic components, pneumatic components, and mechanical linkages or combinations thereof. For examples of such systems, see U.S. Pat. Nos. 4,854,409; 5,193,063; 5,230,528 and 7,222,867.
By way of further example, FIGS. 1 and 2, and a written description thereof, of U.S. Pat. No. 7,222,867, which is assigned to International Truck Intellectual Property Company, LLC., are reproduced herein. A vehicle 10 is shown in FIG. 1 that is comparable to a 8×4 or 8×2 tractor. Vehicle 10 can be any vehicle configured to haul large and varying loads. Vehicle 10 includes a chassis 12 with front and rear fixed axles 14,16, 18, which in tum have wheels 20 mounted thereon to support chassis 12 above a road surface. Chassis 12 carries a body including a driver cab 22 and a cargo body 24, such as a dump body. Because the load carried by vehicle 10 varies greatly it can be advantageous to lower a supplementary axle to avoid having the vehicle violate per axle loading limitations. Here a lift axle 26 is provided as such a supplementary axle. Those skilled in the art understand that full time use of such an axle raises vehicle operating costs due to increased rolling resistance.
Automatic operation of lift axle 26, or, alternatively, giving indication to an operator of appropriate times to raise or lower lift axle 26, involves other vehicle systems which are schematically illustrated in FIG. 2, which discloses a system for a truck that is comparable to 6×2 tractor. Chassis 12 is equipped with an air suspension system in which air filled bladders (air springs 44) take over much if not all of the support and shock isolation functions of conventional solid springs. Among the advantages of air springs is that the quantity of air in them can be adjusted to maintain chassis 12 at a fixed height. To this end an air delivery system works through a height leveling valve 52. Air pressure in the air spring 44 is thus correlated with vehicle load. A pressure sensor 322 is provided for each air spring 44 circuit and provides the basic data for the determination of axle load. Typically there will be only one such circuit per vehicle, however, other arrangements are possible, including individual control for each air spring and intermediate arrangements, such as the two circuit design illustrated in the figure.
Additional suspension stabilizing linkages 66 are associated with each air spring 44 depending from frame side rails 48. Air lines 62 connect to a compressed air tank 68 installed on chassis 12 between side frame rails 48. An engine 70 provides motive power for chassis 12, driving a propeller shaft 76 by an automatic or semi-automatic transmission 72. Propeller shaft 76 is connected between the transmission 72 and a differential 74 for the single drive axle 16 shown. A tachometer 75 is coupled to propeller shaft 76 and allows the determination of the average rotational velocity of the drive 20 wheels from which vehicle speed is estimated. Lift axle 26 is not driven. Pneumatic positioning cylinders 64 are mounted between chassis 12 and lift axle 26 to raise or lower the lift axle as required by the electronic control system.
Heavy trucks, such as shown in FIG. 1, spend much of their time carrying less-than-maximum weight loads. See FIG. 3 where the percentage of usage time is plotted versus vehicle load. This graph indicates that such trucks carry less than their maximum allowable load about 60% of the time they are used. This can be attributed to the nature of their cargo, i.e. certain fleets are “volume limited” rather than “weight limited” (e.g. wood chip haulers). In addition, many trucks lighten their load progressively over the course of their route (e.g. gasoline tankers). Alternatively, other trucks such as dump or garbage trucks may increase their load progressively.
Accordingly, as the data in FIG. 3 indicates, the majority of trucks on the highway would benefit from a tire rolling resistance optimization scheme that takes into account the load placed on tires. More specifically, a method for optimizing the fuel economy of a vehicle that takes into account the rolling resistance characteristics of a tire and appropriately alters the load on the tire would be beneficial. Such a method that uses equipment already present on the vehicle, such as a lift axle, would be particularly advantageous.